THE EXCEniALON. 



61 



riie posterior part of the roof is not converted into nervous 

 matter, but remains thin and attenuated ; the ejje?idy7na, or 

 lining of the cerebral cavity, and the arachnoid^ or serous 

 membrane v,'hich covers the brain externally, coming- nearly 

 into contact, and forming', to all appearance, a single thin 

 membrane, which tears with great readiness, and lays open 

 the cavity of the fourth ventricle. Anteriorly, on the other 

 hand, the roof becomes converted into nervous matter, and 

 may enlarge into a complex mass, which overhangs the 

 posterior division, and is called the cerebellum. The j90??s 

 varolii^ when it exists, is the expression of commissural fibres, 

 which are developed in the sides and floor of the anterior part 

 of the posterior cerebral vesicle, and connect one half of the 

 cerebellum with the other. 



Thus, the hind-brain differs from the posterior cerebral 

 vesicle in being differentiated into the medulla oblongata (or 

 inyelencephcdon) behind, and the cerebellum wdth the pons 

 varolii (which together constitute the metenceplialori) in 

 fi-ont. 



The floor of the middle cerebral vesicle thickens and 

 becomes converted into two great bundles of longitudinal 

 fibres, the crura cerebri. Its roof, divided into two, or four, 

 convexities by a single longitudinal, or a crucial, depression, 

 is converted into the " optic lobes," corpora bigemina or 

 quadrigemina. And these parts, the optic lobes, the crura 

 cerebri, and the interposed cavity, which either retains the 

 form of a ventricle, or is reduced to a mere canal (the iter a 



Pit. 



M.li. 



xsr 



v-^sr 



Vio. 20. — A longitudinal and vertical section of a Vertebrate brain. — The letters as before. 

 The lamina teiininaiis is represented by the strong black line between FM and 3. 



iertlo ad quartum ventricidwn)^ are the components of the 

 mid-brain or Tnesencej^halon. 



The anterior cerebral vesicle undergoes much greater 



